1. Field of the Invention
The present invention relates to a wavelength-division-multiplexing (WDM) optical amplifier, and particularly, to an output control system and output control circuit for individually controlling the levels of optical signals, having different wavelengths, contained in a wavelength-division-multiplexed optical output signal.
A WDM optical transmission system multiplexes optical signals having different wavelengths into a wavelength-division-multiplexed optical signal and transmits the same through an optical fiber. This system is capable of increasing the transmission capacity of each optical fiber depending on the number of optical signals multiplexed.
The optical amplifier for amplifying such a multiplexed optical signal must properly set the output level of each of the optical signals and, when the number of wavelengths of the optical signals is two, minimize the effect of the gains of the amplifier on the optical signals.
2. Description of the Related Art
An optical amplifier employing an erbium (Er)-doped optical fiber is often used because it effectively amplifies the 1.55-.mu.m-band optical signals that are generally used in optical communication. This kind of optical amplifier is useful as a post-amplifier for a transmitter, a preamplifier for a receiver, or an in-line amplifier for a repeater, to extend the transmission distance at low cost.
Wavelength division multiplexing (WDM) effectively increases transmission capacity, and multiplexing 1.55-.mu.m-band optical signals is preferred. To multiplex and demultiplex optical signals of different wavelengths, optical devices must be employed. Employing optical devices, however, causes a loss of transmission distance. To compensate for the loss, optical amplifiers are needed. A combination of WDM and optical amplifiers forms a favorable means to transmit many optical signals over a large distance.
Optical amplifiers usually employ a constant output control technique. This technique is adopted not only for amplifying a single-wavelength optical signal but also for amplifying a wavelength-division-multiplexed optical signal composed of a plurality of optical signals having individual wavelengths, to provide a constant optical output level.
The wavelength-division-multiplexed optical signal usually consists of two optical signals having different wavelengths, one in a 1.535-.mu.m band and the other in a 1.55-.mu.m band. These bands are favorable because erbium-doped optical fibers of the optical amplifier provide high gains on signals in the range of 1.52 .mu.m to 1.54 .mu.m and in the range of 1.54 .mu.m to 1.57 .mu.m.
According to the constant output control technique, the total output level of the amplifier is constant, and therefore, the level of each of the optical signals is obtained by dividing the total output level by the number of the optical signals. Accordingly, the levels of the optical signals may be insufficient when they arrive at a receiver. If one of the optical signals is absent due to some reason, the level of the absent optical signal is distributed to the other optical signals, to increase the levels thereof. This may cause an error at a receiver. On the other hand, if an optical signal having its own wavelength is added, it will decrease the levels of the other optical signals. This is a first problem of the conventional WDM optical amplifier.
The gain of the WDM optical amplifier has wavelength dependency. The wavelength dependency sometimes causes a reception error, which is aggravated by fluctuations in a light source in an optical terminal, in a multiplexer-demultiplexer, or in transmission fibers. Namely, these fluctuations change the levels of the optical signals contained in a wavelength-division-multiplexed optical signal at a receiver. Then, the receiver cannot detect every optical signal. In particular, any optical signal at a low level easily causes a reception error. This is a second problem of the conventional WDM optical amplifier.
The WDM optical amplifier of the prior art prepares a monitor signal and checks the level of the monitor signal to keep the optical output of the amplifier constant. Due to the second problem mentioned above, the gain of the amplifier for the optical output is not always equal to that for the monitor signal. This results in fluctuations in the optical output of the amplifier. Preparing the monitor signal involves additional costs. The monitor signal will not be a true representative of the optical output of the amplifier if the system for preparing the monitor signal fails.
The WDM optical amplifier of the prior art involves a gain tilt. When collectively amplifying two optical signals with one excitation beam, the optical amplifier causes a gain tilt to weaken one optical signal relative to the other depending on the wavelength of the excitation beam, although the amplifier uniformly amplifies a single optical signal.